# std::set<Key,Compare,Allocator>::swap

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< cpp‎ | container‎ | set

void swap( set& other ); |  | (until C++17)
void swap( set& other ) noexcept(/* see below */); |  | (since C++17)

Exchanges the contents of the container with those of other. Does not invoke any move, copy, or swap operations on individual elements.

All iterators and references remain valid. The `end()` iterator is invalidated. The `Compare` objects must be Swappable, and they are exchanged using unqualified call to non-member `swap`.

If std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then the allocators are exchanged using an unqualified call to non-member `swap`. Otherwise, they are not swapped (and if get_allocator() != other.get_allocator(), the behavior is undefined). | (since C++11)

### Parameters

other | - | container to exchange the contents with

### Exceptions

Any exception thrown by the swap of the `Compare` objects. | (until C++17)
`noexcept` specification: noexcept(std::allocator_traits<Allocator>::is_always_equal::value && std::is_nothrow_swappable<Compare>::value) | (since C++17)

### Complexity

Constant.

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <set>

    template<class Os, class Co>
    Os& operator<<(Os& os, const Co& co)
    {
        os << '{';
        for (auto const& i : co)
            os << ' ' << i;
        return os << " } ";
    }

    int main()
    {
        std::set<int> a1{3, 1, 3, 2}, a2{5, 4, 5};

        auto it1 = std::next(a1.begin());
        auto it2 = std::next(a2.begin());

        const int& ref1 = *(a1.begin());
        const int& ref2 = *(a2.begin());

        std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
        a1.swap(a2);
        std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';

        // Note that every iterator referring to an element in one container before the swap
        // refers to the same element in the other container after the swap. Same is true
        // for references.

        struct Cmp : std::less<int>
        {
            int id{};
            Cmp(int i) : id{i} {}
        };

        std::set<int, Cmp> s1{{2, 2, 1, 1}, Cmp{6}}, s2{{4, 4, 3, 3}, Cmp{9}};

        std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';
        s1.swap(s2);
        std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';

        // So, comparator objects (Cmp) are also exchanged after the swap.
    }

Output:

    { 1 2 3 } { 4 5 } 2 5 1 4
    { 4 5 } { 1 2 3 } 2 5 1 4
    { 1 2 } { 3 4 } 6 9
    { 3 4 } { 1 2 } 9 6

### See also

std::swap(std::set) | specializes the std::swap algorithm (function template)

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